Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for refining positions of marine seismic receivers that move with ocean currents.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. This image is generated based on recorded seismic data. The recorded seismic data includes pressure and/or particle motion related data associated with the propagation of a seismic wave through the earth. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process. The image illustrates various layers that form the surveyed subsurface of the earth.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in determining the above-noted reservoirs. Marine reflection seismology is based on using a controlled source of energy that sends the energy (seismic waves) into the earth. By measuring the time it takes for the reflections and/or refractions to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off the geological structures present in the subsurface includes a vessel that tows an array of seismic receivers provided on streamers. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface. The streamers may have other than horizontal spatial arrangements. The vessel also tows a seismic source array configured to generate a seismic wave. The seismic wave propagates downward and penetrates the seafloor until eventually a reflecting structure (reflector) reflects the seismic wave. The reflected seismic wave propagates upward until detected by the receiver(s) on the streamer(s). Based on the data collected by the receiver(s), an image of the subsurface is generated.
However, this traditional configuration is expensive because the cost of streamers is high. Further, due to the great length of the streamers, e.g., 10 km, the streamer array is difficult to maneuver around various obstacles, e.g., an oil platform. New technologies deploy plural seismic sensors on the bottom of the ocean (ocean bottom stations) to avoid this problem. Even so, positioning the seismic sensors remains a challenge.
Such technologies use permanent receivers set on the ocean bottom, as disclosed in U.S. Pat. No. 6,932,185, the entire content of which is incorporated herein by reference. In this case, the seismic sensors are attached to a heavy pedestal. A station that includes the sensors is launched from a vessel and arrives, due to its gravity, at a desired position and remains on the bottom of the ocean permanently. Data recorded by sensors is transferred through a cable to a mobile station. When necessary, the mobile station may be brought to the surface for data retrieval.
Although the ocean bottom nodes better handle the various obstacles present in the water, using them is still expensive and difficult because the sensors and corresponding pedestals are left on the seafloor. Further, positioning the ocean bottom nodes is not straightforward.
An improved approach to these problems is the use of plural (e.g., thousands) autonomous underwater vehicles (AUVs) for carrying the seismic sensors and collecting the seismic data. The AUVs may be launched from a deployment vessel, guided to a final destination in the ocean, instructed to record the seismic data, and then instructed to surface for retrieval. Such a system is disclosed in U.S. Pat. No. 9,417,351, which is assigned to the assignee of the present application. However, many challenges remain with the use of a large number of AUVs for collecting seismic data. One such challenge is correct positioning of the AUVs because measuring and/or predicting the positions of underwater AUVs is still challenging.
Accordingly, it would be desirable to have systems and methods that refine/improve the positions of underwater receivers participating in a seismic survey.